Process for developing electrostatic latent images



United States Patent 7 Claims. or. 96-11) The invention relates to aprocess for the production of electrophotographic images, using liquiddevelopers.

The principle of the known electrophotographic processes consists inthat a latent, electrostatic charging image is produced by image-wiseexposure of a. charged photoconductive layer, which image is thendeveloped by a developer in liquid or powder form which is precipitatedaccording to the charge on the photoconductive layer, to

form a visible image. The known powder developers consist of particlesof fusible resins, which contain pigments such as carbon black and ofcarrier materials such as quartz sand. When these mixtures are used, itis necessary to fix the visible image by heating the image until thepigmented resin particles fuse on the support.

It is also known to develop a latent charging image by spraying withdyed aerosols, the charged Zones on the electrophotographic materialbeing coloured. The liquid which is dried or absorbed thereon thenproduces the stable image. According to an embodiment of these processesthe aerosol is charged electrostatically for example by means of a highvoltage installation, and is sprayed into the electrical field of anelectrode system in which the electrophotographic material constitutesan electrode. Furthermore the electrophotographic layer can be developedin the corresponding manner with un coloured liquid mists. In thesecases, the liquid image is transferred to a second image support and ismade visible on the latter by means of soluble dyestuffs.

According to other known processes, the surface which is chargedaccording to an image is developed with a developer liquid.

The known processes can be improved by the deposition of the developerliquid being controlled by means of an electric field, which is set upbetween the electrophotographic material bearing the, latentelectrostatic image and an electrode facing this layer. Such anarrangement is more especially absolutely necessary for aerosoldevelopment.

For the developing methods which are based on electrophoretic principlesthe developing liquids consist of a dispersion of the colour-supplyingcomponent in a neutral organic liquid with high electrical resistivityand low dielectric constant. Suitable liquids include kerosene,turpentine, benzene or carbontetrachloride; they are selected because oftheir high resistivity, a property which is essential to avoid dischargeof the latent electrostatic image. The coloured or colour-supplyingdeveloper particles dispersed in said isolating liquid have as regardstheir electric properties to meet the condition that their relaxationtime is far greater than the developing time. In many cases, the liquiddevelopers also have added thereto artifical or natural resins, whichimpart a charging opposite to the latent image to the dispersedcolouring particles or by means of which the pigment is fixed on theimage support when the liquid medium has evaporated. is accomplished bythe aid of an electric field between two electrodes one of which is theelectrophotographic In the use of these developers the developmentmaterial respectively a metal plate on which said material is arranged.The colour component dispersed in the high isolating developer liquid istransported in said electric field to the latent electrostatic image ofsaid electrophotographic material forming thereby a visible image. Inthis case in the course of the development a change of the concentrationof the coloured developer particles dispersed in the developing liquidoccurs in such a way that the coloured particles are concentrated in thevicinity respectively on the electrophotographic layer. The developingliquid acts only as carrier medium for the coloured particles and doesnot actively take part in the development.

The known processes have disadvantages in many respects, so that the usethereof in practice is questionable. The organic support liquids whichcan be used are generally combustible and physiologically are notunobjectionable. In order to prevent injury to the personnel workingwith these developers, it is consequently often necessary to provideextensive protective measures. An

' additional disadvantage is that these organic liquids act the organicdeveloper liquids are partially lost with the development, thus thesedeveloping processes are also often very uneconomic.

In developing methods working according to electrolytic principles alsotransport of the coloured developer particles and change ofconcentration of said particles in the developer liquid occurs in thecourse of development. The developer liquids consist generally of asolution of the coloured or colour-supplying component in a suitablesolvent. The liquid acts only as a carrier medium for the developerparticles and does not actively take part in the development.

The electrolytic developing processes are disadvantageous in manyrespects. The developing times are too long since the electrolyticdeposition of the developing particles requires times in the order ofmagnitude of minutes. Furthermore in this process a final drying of theelectrophotographic materials is necessary since said materials aresucked up with developer liquid. The electrophotographic materialssuitable for this process have to contain a layer of a conductive metale.g. aluminium.

summarising, it can be established that the processes referred to arecharacterised by the relatively high technical expense which isnecessary and by an increased uncertainty which is caused thereby.

It is among the objects of the instant invention to avoid thedisadvantages described above and to provide a simple and inexpensivetechnique for developing electrostatic images. The developing methodaccording to the invention makes use of a pyhsical phenomenon whichhitherto was not used in connection with electrophotographic processes.The new process is characterized by the terrn wetting development.

It has now surprisingly been found that an electrophotographic layer.which is imagewisely charged is be left to dry on the support or can betransferred by printing to a second support in known manner while stillmoist.

The aqueous developers suitable for the process according to theinvention have to meet the following conditions: The difference of thewetting angle between the light-struck and nonlight-struck areasrespectively the charged and the uncharged areas of the exposedelectrophotographic layer should be as great as possible whereby thewetting angle at those areas of the latent electrostatic image not to bewetted should be greater than approximately 90 and at those areas to bewetted smaller than 90. The wetting angle can be measured according toknown methods or can be estimated in usual manner from the interfacialenergies according to the Youngequation. The magnitude of theinterfacial energy at a liquid-gas, liquid-liquid or liquid-solidinterface is closely related to the surface tension of the liquid onthat interface.

With regard to the electric properties of the aqueous developer inparticular the so-called relaxation time is important. Under relaxationtime is to be understood the time necessary for the displacement ofcharges inside the developer drop or developer film under the in fluenceof an electric field. In the case of the wetting development therelaxation time shall be small as compared with the developing time. Ifso, the aqueous developer has to be considered as conductive. Therelaxation time can be estimated from the following equation In thisequation 1 represents the relaxation time, s the absolute dielectricconstant, e the dielectric constant of the aqueous developer and p theresistivity. Hence it follows that the relaxation time can mainly beinfluenced within wide limits by varying the conductivity of thedeveloper liquid. Aqueous developers having a resistivity smaller thanohm-cm. are preferably suitable for the wetting development.

Aqueous developers suitable for the process according to the inventioninclude solutions or dispersions of coloured compounds or compoundscapable of yielding coloured products by subsequent chemical reaction.Suitable coloured products are for example (the numbers in parenthesesrepresent the numbers of the corresponding dyestuifs in the Schulzdyestutf tables):

Water-soluble azo dyestuffs, such as Azofuchsin B (80), Echtrot ET(122), Benzogrun C (668), Bismarckbraun (311), Naphthylamin 10B (299),Viktorias-chwarz B (581); triaryhnethane dyestuffs, such asMalachitgriin (754), Kristallviolett (785), Saurefuchsin (800),Tiirkisblau BB (757); diphenyl methane dyestuffs, e.g. Auramin G (753);stilbene dyestuffs, such as Diphenyl orange RR (710) or Brillantgelb(724); pyrazolone deystuifs, such as Echtlichtgelb G (732), Tar-trazin(737); xanthene dystuifs, such as Rhodarnin SG (862), Saureviolett 4R(871), Eosin (881), Erythrosin (886), Phloxin (888), Rose bengale (889,891); acridine dyestuffs, e.g. Acridingelb (901) or Acridinorange (902);quinoline dyestuffs, such as Chinolingelb (918); quinonimine dyestuffs,such as Tolylenblau (937), Hansagriin (940); thiazine dyestuffs, e.g.Methylenblau (1038), Methylengriin (1040), thiazole dyestuffs, e.g.Primulingelb (932); azine dyestuffs, e.g. Indulinscharlack (947),Phenosafranin (958), Nigrosin (986); osazine dyestuffs, such asCoeles-tin-blau B (1015); sulphur dyestuffs, such as Thiongriin B (116).

It is obvious that mixtures of such dyestuffs are also suitable ascolour-yielding component.

It is also possible to use coloured water-soluble binary compounds orcomplexes, such as form a number of transition elements, primarily ofthe first long period of the Periodic System, for example the knowncuprous tetrammine complex, chromium salts such as chrornic sulphate,potassium chromealum, potassium chromate, compounds known as ammine,aquo and acido com- 4 plexes of trivalent chromium, potassiumpermanganate, ferric compounds, such as ferric thiocyanate and thethiocyanato ferrates, soluble Berlin blue the Prusside compounds ofiron, acetato ferric salts, iron-ammonium citrate, the thiocyanate andthe thiocyanocobaltates of divalent cobalt, cobaltous sulphate, andchloride, the large number of cobaltic complexes, for example theammine, aquo and acido complexes, which exist in similar abundance tothose of the chromous complexes, chlorides and sulphates of divalentnickel, the copper tartratecomplex, copper glycine, the solublecompounds between iron and gallic acid or tannin, the complexes of theferrous salts with a-picolinic acid or analogous compounds which containa cyclically combined nitrogen atom in the a-position to a carboxylgroup, the complexes of divalent iron or cobalt with ot-diOXiIIlCS, suchas dimethyl glyoxime, the ferric complexes with salicylic acid, thecompounds between titanium or iron salts and pyrocatechol orchromotropic acid.

Since aqueous solutions as well as dispersions of preferably colouredcompounds can be used as developer liquids for the developing processaccording to the invention it is easily possible to select a suitablecompound from the large number of available coloured products. It shouldbe expressly pointed out that the chemi cal structure of the colouredcompound is not especially critical and that the compound is selectedaccording to the requirements of the reproduction process in which thedeveloper is to be used.

It is apparent from what has been shown above that all water-compatiblecolouring compounds are suitable for the present purpose.

According to another embodiment of the invention the development can beaccomplished with colourless aqueous solutions if the developer hasdissolved therein substances which supply coloured products by achemical reaction. 7

v with alteration of the pH-value.

Many different types of reactions can be used for this process. Forexample, the aqueous developer can have added thereto substances whichshow a change in colour In principle, this group of substances includesall compounds which are used in known manner as indicators. It ispreferred to use thosesubstances which per se are colourless and changeinto a coloured form with a change in the pH-value. The substance is allthe more suitable in proportion as the corresponding coloured product ismore intensively and deeply coloured. It is also possible to usereduction reactions or exchange reactions which lead to the formation ofcoloured inorganic or organic compounds. In practice, it is possible toemploy all reactions which are known from analytical chemistry and servefor detecting certain cations or anions for example the formation ofcoloured metal sulphides.

Coupling reactions which use diazonium compounds and which lead to theformation of coloured products are especially suitable.

The possibility of using chemical reactions for the coloured developmentof electrophotographic images with the use of developers in powder formis already known. In contrast hereto, the process according to theinvention does however have considerable advantages. Thus, the choice ofsuitable reactants is substantially simpler, since in the present casethe only condition as regards whether they can be use-d is thecompatibility with water of at least one reactant and the formation of areaction pro-duct of deepest possible colour. With the known developers,on the other hand, as well as the properties necessary for the chemicalreaction, it is also necessary to observe the electrostatic ortriboelectric properties necessary for a developer in powder form.Furthermore, in these cases, additional steps such as for examplemoistening, heating and the like are usually also necessary forinitiating the colour reaction.

The reaction components for the colour-yielding reaction can beintroduced in various ways. The simplest possiblity of reaction isprovided by incorporating one of the components in theelectrophotographic layer, the other component being dissolved in thedeveloper. The reaction then takes place in the regions of theelectrophot-ographic material, which is wetted according to the processof the invention by the aqueous developer. With this form of theprocess, the component in the electrophotographic layer and reactingwith. the developer solution can simultaneously act as sensitiser forthe photoconductor or it can even itself be the photo-conductivesubstance.

It is also possible to dissolve two colourless substances in thedeveloper, which substances only participate in a reaction withformation of a coloured product upon contact with theelectrophotographic layer, either by catalysts present therein or byphysical means.

Furthermore, when using conventional electrophotographic materialscontaining no additional substances, one of the reaction components canbe dissolved in the developer, be developed in the manner according tothe invention and thereafter the colour formation can be achieved in anadditional working step by treatment with a liquid or gaseous reactioncomponent. 7

The above statements illustrate that the development process accordingto the invention is capable of being varied in many ways. The choice ofa specific colour reaction will depend substantially on the requirementsof the printing process actually being used.

It has proved to be advantageous that the image-wise chargedelectrophotographic image is not dipped into the aqueous developer butis merelly brought into contact with the surface of the aqueousdeveloper. According to a preferred embodiment the development isachieved by moving the electrophotographic material containing thelatent electrostatic image over a film of the developer liquid in such amanner that one section of the surface after the other is wetted by theliquid film.

The liquid film should with advantage be screened. Various possibilitiesexist for the production of such a liquid screen. Thus, the liquid levelcan be covered with a fine-mesh, non-absorbing fabric, consisting forexample of polyamides. The developer liquid will then be brought intocontact with the surface of the image-wise charged electrophotographiclayer through the meshes of the fabric and Wet the said layer accordingto the charging. By the screen like fabric a uniform application of theaqueous developer is simultaneously obtained. Furthermore it is possibleto achieve a weaker or :stronger application of the possibly coloureddeveloper liquid by varying the force with which the electrophotographiclayer is pressed onto the fabric containing the developer. Instead ofthe fabric which has been described, it is also possible to use devicesor rollers provided with a roughened screen-like surface.

These devices should consist of materials which are wetted by thedeveloper liquid without however absorbing the said liquid. Furthermore,.a certain elasticity is necessary so that the liquid level can beadapted to the surface of the electrophotographic layer bearing thelatent electrostatic image.

' Synthetic plastics are therefore especially suitable for thesedevices.

The developing apparatus shall preferably contain devices or'means forstoring a sufiicient quantity of developer. This can be achieved forexample by a sand cushion sucked up with the developer liquid.Furthermore, it is possible to use roller systems such as from theprinting industry. In this case, the applicator roller provided with ascreen-like notched pattern is supplied with the necessary liquid bymeans of a second roller from a developer storage container.

The liquid image is generally absorbed into the microscopically smallirregularities of the surface of the image support thus producing animage being fast to wiping. In cases which make it desirable to have aparticularly high mechanical resistance of the image, the developersolution can contain adhesive or thickening agents added thereto withoutany disadvantages as regards the quality of the developed image, forexample gum arabic, agaragar, tragacanth, dextrins, guar, carraghenates,cellulose derivatives such as ester or ether, alkali salts or esters ofalginic acid, polyvinyl alcohol, polyvinyl pyrrolidone, polyvinylcarbazole and the like. The properties of the aqueous developer can bemodified by the addition of alcohols such as glycols, glycerole orpolyethylene oxides.

By comparison with the known development processes, the processaccording to the invention has numerous advantages. In addition toeconomy, there is especially to be mentioned the safe handling of theaqueous solution. Furthermore, a charging of the developer such as isnecessary with the various forms of aerosol development is superfluous.The electrophotographic images which are obtained are superior asregards quality to those produced by the known development processes.Since no pigment particles are deposited, the images are without grainand very sharp, so that these images can be greatly enlarged. The liquidis absorbed onto the support in a manner similar to ink, so that aseparate fixing, as for example with the various powder processes, isunnecessary. The process according to the invention also makes itpossible for development with different colours to be carried out insuccession.

Example 1 320 g. of silicone resin (for example type Bayer P150), 60% intoluene, 500 g. of toluene and 450 g. of chemically pure,photoconducting zinc oxide are ground for about 3 hours in a ball mill,thereafter cast on to paper and dried. The electrophotographic paperthus produced is electrostatically charged by means of a coronadischarge device and exposed to form an image. For the development ofthe latent charging image, a filtered solution of 500 mg. ofKristallviolett in 10 ml. of water is used. The electrophotographicimage support is for this purpose tensioned on a roller with a thicknessof about 10 cm. and rolled over a sand surface which is formed into apaste with the dyestulf solution and which is spread out flat anddoctored. The grain size of the sand is between 25 and 50 A sharp,practically grainless, violet colour image is obtained, which adheresafter a few seconds to the support in such a way that it cannot be wipedolf.

Example 2 The same material as in Example 1 is used. The latent image isdeveloped in the manner described with a 3% aqueous potassiumpermanganate solution. After. drying, a vigorous brownish image isobtained.

Example 3 300 g. of a product which has been prepared by vacuumesterification of 2 mols of phthalic acid anhydride, 1 mol of adipicacid, trimethylol propane and subsequent reaction with 4.8 mols ofcyclohexyl isocyanate are dissolved in 1000 g. of acetic ester. 670 g.of photocon ductive zinc oxide are then added and the mixture is shakenfor 2 hours on a vibratory mill, cast on to barytacoated paper anddried. An electrostatic charging image is produced on the layer by knownmethods. In order to make the image visible, a developer liquid preparedby dissolving mg. of Bengalrosa in 20 ml. of water is applied by meansof a roughened steatite roller. The surface structure of the roller hasan average spacing of the irregularities of about 10 to 500 1. with adepth of 30 to 100p.

Example 4 A mixture of 140 g. of photoconductive zinc oxide 120 g. ofalkyd resin (for example Alkyd SM 50, trade name of FarbenfabrikenBayer), 320 g. of toluene and 10- g. of

cobalt naphthenate solution (10% in toluene) is applied to aluminiumlined paper and dried. The development of the charged layer exposed toform an image is effected with an aqueous solution of PapiertiefschwarzAGX (trade name of Farbenfabriken Bayer) which contains 1.5 g. ofdyestuif to 300 ml. of water. The dyestuff solu tion is applied by meansof a rubber roller, which is covered with Perlon mesh fabric. The meshfabric has a thickness of about 50 1. and a mesh size of about 2511.. Adeep black image is obtained.

Example 5 The electrophotographic material described in Example 4 isused. The liquid used for the development consists of a solution of 11.5g. of tannin, 3.8 g. of crystallised galli-c acid, 15 g. of ferroussulphate, 5 g. of 25% hydrochloric acid, 0.5 g. of phenol and 5 g. ofgum arabic, in 500 cc. of water. The electrostatic charging image isproduced according to Example 4 and made visible, and immediately aftercompleting the developing process, it is transferred to normal writingpaper. A copy of the original corresponding exactly to the document isob tained.

Example 6 200 g. of white, photoconductive zinc oxide are dispersed in300 g. of a 35% solution of polystyrene in toluene by means of ahigh-speed mixer, cast on to an acetyl cellulose film vapour-coated withaluminium and dried. The latent charging image produced on the layer isdeveloped with the aid of aqueous iron-ammonium citrate solution. Forthis purpose, the image support is pressed on to a cushion, the cover ofwhich consists of a nylon fabric with a mesh size of an average 200 andfilled with polystyrene beads with a size of 300 to 400;. On strippingoff the image support an image of rusty brown colour is obtained.

Example 7 The material described in Example 6 is used. As developerliquid, there is employed a clear solution of 14 g. of 80% tannin, 3.5g. of crystallised gallic acid, g. of 15% FeCl solution, 4 g. ofhydrochloric acid (1.16), 2.5 g. of naphtholblauschwarz, 0.5 g. ofphenol in 500 g. of water. A bluish black image is obtained.

Example 8 100 g. of chemically pure photoconductive zinc oxide (Merckp.a.), 30 ml. of silicone resin, 60% in toluene (for example siliconeresin Bayer P150) and 100 ml. of toluene are introduced into a ball mill(capacity 1 litre), which is half filled with porcelain balls (1.5 cm.diameter). The mixture is ground for 2 hours and thereafter applied bymeans of an applicator roller to document paper (100 g./m. and dried.The thickness of the layer is about 1511.. The material iselectrostatically charged at 7 'kv. by means of a corona dischargedevice, exposed image-wise by means of an incandescent lamp anddeveloped as follows.

For preparing the developer, the ultra-fine fraction of the normalcommercial quality of carbon black known as Spezialschwarz IV (Degussa)is formed into a paste with concentrated nitric acid, diluted withdistilled water until of the consistency of honey and thereaftercarefully heated as long as nitrous gases are evolved. Thereafter, it isdecanted several times with water and thereafter boiled with strongsodium hydroxide solution for 30 minutes. In this way, a carbon blacksuitable as developer pigment is obtained in ultra-fine dispersion. 5parts by weight of this carbon black are dispersed in 100 parts byweight of an aqueous solution of wax-free shellac and borax. Thesolution is prepared by 50 g. of borax and g. of powdered white shellacbeing allowed to swell in 1 litre of water and being thereafter heateduntil dissolved.

In accordance with the simplest embodiment of the development process,the developer liquid is taken up with a long-haired broad bristle brushand the brush is moved in one stroke over the previously preparedelectrophotographic layer. The developer liquid remains adhering to thecharged image areas and after drying, supplies a black, water-resistantreproduction of the exposed original, which reproduction cannot be wipedoff. In accordance with another embodiment, a roughened soapstone rolleris used for applying the developer liquid, the surface structure of saidroller having a mean spacing of the irregularities of about 10 to 500with a depth of 30 to 100;/..

Example 9 2 g. of the developer pigment prepared according to Example 8are dispersed in 100 ml. of a 10% aqueous solution of polyvinylpyrrolidone and is used in this form for wetting the layer provided withan electrostatic charging image. For the production of the aqueoussolution, the product obtainable commercially under the name LuviscolK30 is used. The developer liquid is applied by one of the methodspreviously described. A deep black colour image is formed, but incontrast to the colour image produced according to Example 8, it canstill be washed off. The colur density of the image can be changed insimple manner by diluting the developer solution with water.

Example 10 The electrophotographic material described in Example 8 isdeveloped with an aqueous suspension of carbon black which contains abinding agent. This suspension is applied by one of the previouslydescribed methods to the prepared electrophotographic layer. It providesa black image of the original which cannot be washed off.

Example 11 The process is performed as described in Examples 8 to 10with the exception that for the developing of the photoconductive layerbearing the electrostatic image is used an aqueous suspension of cobaltaluminate, containing a water-soluble binding agent. The dye paste isdiluted with water to obtain a suitable viscosity and the required colordensity. The resulting final image is characterized by an excellentfastness to light.

The cobalt aluminate of the dye suspension described above can bereplaced with one of the following dyes: Alizarine-krapplack, which isdescribed in the handbook Technologie der Textilfasern by Fierz-David,vol. 3, Kiinstliche organische Farbstoffe, 1926, page 524 fol lowing,green chromium oxide, sienna, which is a brownish-yellow clay, which isdescribed, for example, in Hackhs Chemical Dictionary, 3d edition, page769, or cadmium sulfide.

In order to make the water image produced by the process of theinvention legible, the developer liquid water has added thereto awater-insoluble dyestuff and a water-soluble binder, which bonds thedyestuif to the final support after evaporation of the water. Many suchaqueous dyestuff dispersions containing a binder are Example 12 anapplicator roller to document paper (100 g./m. and

dried. The thickness of the layer is about 15;/.. The material is nowelectrostatically charged by means of a corona discharge device at 7kv., image-wise exposed with incandescent lamp light and developed with15% sodium hydroxide solution. For the application of the developerliquid, there is employed a roughened soapstone roller, the surfacestructure of which has a mean spacing of the irregularities of about 10to 500 with a depth of 30 to 100;. A reddish-violet image is formed onthe white layer.

Example 13 The electrophotographic material described in Example 12 isused, but Without containing any phenol phthalein. The material isexposed to light in the manner indicated above and thereafter developedwith an aqueous acid bromophenol blue solution, which is pale yellow incolour. Thereafter, the developed electrophotographic paper is treatedwith gaseous ammonia, a bluish- 7 black image being formed.

The bromophenol blue can also be replaced by bromocresol green.

' Example 14 The electrophotographic layer material is prepared bymixing 150 g. of electrophotographic zinc-oxide, 100 g. of siliconeresin, 150 ml. of toluene and a dyestulf solution consisting of 0.05 g.of Bengal pink in 6.5 ml. of methyl alcohol. A paper support is coatedwith this mixture and an electrophotographic material is obtained whichhas a sensitis ation maximum in the region of 575 m The processing iscarried out in accordance with Example 12. Formic acid, diluted in theratio 1:1 with water, is used as developer liquid. The solution bleachesout the sensitising dyestuif at the charged image areas and a negativeimage of the original becomes visible.

Example 15 Example 13 is modified by incorporating 0.02 g. of crystalviolet as sensitising dyestuff into the coating material. Development ofthe layer with hydrazine hydrate (25%) yields a white image on a blueground.

Example 16 100 g. ofzinc oxide, 30 ml. of silicone resin solution (60%in toluene), 100 ml. of toluene and 10 g. of lead acetate are trituratedfor approximately 2 hours in a ball mill in order to obtain a uniformsmooth consistency of the constituents. The mixture is then cast onbarytacoated paper with a layer thickness of about 10a and dried. Inaccordance with the foregoing examples, an electrostatic image isproduced on the layer and this is developed with a saturated solution ofthioacetamide in 10% sodium hydroxide solution. For applying thedeveloper solution, a rubber roller is employed which is covered withperlon mesh fabric. The mesh fabric has a thickness of about 50a and amesh size of on average 25a. The alkali thioacetamide solutionpenetrates at the wetted image areas into the layer and reacts with thelead acetate to form a black lead sulphide image. A sodium sulphidesolution can be used instead of the thioacetamide solution with equalsuccess.

Example 17 The same electrophotographic material as described in Example13 is used and this is exposed to form an image and developed with anaqueous solution of cobalt a bluish black image.

amples:

chloride. In this way, a practically invisible image consisting ofcobalt chloride solution is obtained. If the image is exposed to gaseoushydrogen sulphide, a deep black reproduction of the subject is obtained.

Example 18 Example 19 5 g. of the ZnCl -double salt of diazotisedS-benzoylamino-2-amino-1,4-hydroquinone diethyl ether are added asreaction component to the coating material according to Example 15. Theelectrostatic charging image is developed by the process described inExample 15, using a 10% a-naphthol solution in sodium hydroxide solution(10%). The coupling reaction of the reaction components in the layer andin the developer solution leads to In a manner similar to that indicatedabove, it is also possible to use the double salts of other diazotisedaro-' matic amino compounds, of which the following are H-(HO-OHr-CHQzN-QNH,

CHrGL Ta Example 20 An electrostatic charging image is produced on anelectrophotographic layer, prepared by applying a mixture of g. ofsilicone resin, 60% in toluene, g. of zinc oxide and g. of toluene toaluminium-lined paper. The charging image is thereafter developed by theprocess described in Example 12 with a solution which is prepared fromtwo parts of a solution of 0.25 g. of diethyl aniline (mono-free) in 200ml. of sulphuric acid 1:1 and 3 parts of a solution of 2 g. of potassiumhexacyanoferrate (III) in 100 ml. of water. A brownish-red image of theoriginal appears on the zinc oxide layer.

Example 21 An electrophotographic ZnO-layer is charged electrostaticallyby means of a corona discharge device and exposed to form an image. Thea-naphthol solution described in Example 17 is used as developer and itis applied in the same way. The still moist liquid image is transferredafter development to a sheet of paper, which has been coated with aaqueous solution of the diazotised hydroquinone diethyl ether mentionedin Ex ample 6 and thereafter dried. After pulling the two papers apart,the transfer sheet bears a blackish-blue image of the original.

Example 22 An electrophotographic ZnO paper is treated as inthepreceding example and developed with a 10% aqueous FeCl -solution.with saturated alcoholic gallic acid, a black image of the original isformed on the transfer sheet.

Example 23 An electrophotographic material is used, which corresponds tothat used in Example 19, but has an additional content of a-naphthol.The material is processed in accordance with Example 19 and developedwith a 10% sodium hydroxide solution. The material becomes alkaline atthe wetted zones and thereby initiates the coupling reaction, whichleads to a bluish-black dyestuff.

Having thus described our invention we now believe our invention to becapable of numerous variations in methods, apparatus and materials. Forexample zinc oxide as well as all other known inorganic and organicphoto conductive substances can be used e.g. cadmium sulphide,

. selenium, sulphur, arsenic trisulphide, lead iodide, lead chromate,cadmium iodide, mercury iodide, aluminium iodide, and also anthracene,anthraquinone, acenaphthene, chyrysene, p-diphenylbenzene, benzanthrone,1,5- dicyanonaphthalene, 1,4-dicyanonaphthalene, aminophthalodinitrile,nitrophthalodinitrile, as well as photoconductive azomethines,oxazolones, oxodiazoles, triazoles, imidazolones, imidazolthiones, andin addition polyvinyl carbazoles or other polymeric substances havingsimultaneously film forming properties.

The invention can also be employed for the development ofelectroradiographic images. The latent electrostatic image to bedeveloped is in this case produced by means of a xero-radiographicmaterial known per se, the photoconductive layer of which contains forexample selenium, zinc oxide, lead iodide or cadmium sulphide sensitiveto X-rays.

Furthermore, it is possible to use the process according to theinvention for the development of those electrophotographic images whichhave been recorded on insulating foils.

What we claim is:

1. In a method of producing electrophotographic images which comprisescharging a photoconductive After transfer to a paper impregnated layer,exposing the charged photoconductive layer to an object to form a latentelectrostatic image of the object, and developing the exposedphotoconductive layer, the improvement comprising developing thephotoconductive layer bearing the latent electrostatic image byuniformly contacting the entire layer with an aqueous composition havingelectric and wetting properties sufficient to wet the charged areas ofthe photoconductive layer while being repelled from the uncharged areasof the photoconductive layer, and said aqueous developer compositionhaving a relaxation time which is small compared with the developingtime and having a resistivity smaller than 10 ohm-cm. and having wettingproperties wherein the wetting angle measured on the photographic layerof the aqueous developer is greater than degrees at the uncharged areasand smaller than 90 degrees at the charged areas.

2. A method according to claim 1 wherein the contacting of the layerwith the composition is accomplished by means of a mesh arrangement tobreak-up the actual I contact in discontinuous fashion.

veloper composition contains a colourless reaction component for colourforming chemical reaction while the compound reacting with said reactioncomponent is present in the photoconductive layer.

7. A method according to claim 1 wherein the photoconductive layer iscontacted with the aqueous developer by means of a roller having ascreen-like surface.

References Cited by the Examiner UNITED STATES PATENTS 2,221,776 11/1940Carlson 11737 X 3,001,888 9/1961 Metcalfe et a1. 25262.1 X 3,010,88311/1961 Johnson et al 204l8 3,068,115 12/1962 Gundlach 961 X 3,072,5411/1963 Shely et al. 20418 3,081,263 3/ 1963 Metcalfe et a1. 9613,083,117 3/1963 Schmiedel 252-.62.1 X 3,084,043 4/ 1963 Gundlach 9613,102,045 8/ 1963 Metcalfe et a1. 96-1 X 3,172,827 3/1965 Tulagin et al96-1 X NORMAN G. TORCHIN, Primary Examiner.

A. LIBERMAN, D. D. PRICE, Assistant Examiners.

1. IN A METHOD OF PRODUCING A ELECTROPHOTOGRAPHIC IMAGES WHICH COMPRISESCHARGING A PHOTOCONDUCTIVE LAYER, EXPOSING THE CHARGED PHOTOCONDUCTIVELAYER TO AN OBJECT TO FORM A LATENT ELECTROSTATIC IMAGE OF THE OBJECT,AND DEVELOPING THE EXPOSED PHOTOCONDUCTIVE LAYER, THE IMPROVEMENTCOMPRISING DEVELOPING THE PHOTOCONDUCTIVE LAYER BEARING THE LATENTELECTROSTATIC IMAGE BY UNIFORMLY CONTACTING THE ENTIRE LAYER WITH ANAQUEOUS COMPOSITION HAVING ELECTRIC AND WETTING PROPERTIES SUFFICIENT TOWET THE CHARGED AREAS OF THE PHOTOCONDUCTIVE LAYER WHILE BEING REPELLEDFROM THE UNCHARGED AREAS OF THE PHOTOCONDUCTIVE LAYER, AND SAID AQUEOUSDEVELOPER COMPOSITION HAVING A RELAXATION TIME WHICH IS SMALL COMPAREDWITH THE DEVELOPING TIME AND HAVING A RESISTIVITY SMALLER THAN 10**6OHM-CM. AND HAVING WETTING PROPERTIES WHEREIN THE WETTING ANGLE MEASUREDON THE PHOTOGRAPHIC LAYER OF THE AQUEOUS DEVELOPER IS GREATER THAN 90DEGREES AT THE UNCHARGED AREAS AND SMALLER THAN 90 DEGREES AT THECHARGED AREAS.